Centrifuges are widely used throughout the world to separate materials having different properties. Chief among the types of centrifuges are ones with imperforate baskets and ones with perforate baskets. Centrifuges with imperforate baskets are commonly referred to as solid bowl centrifuges and are typically used to separate materials of different densities. In operation, the high speed of the rotating centrifuge centrifugally flings the denser materials to the outer regions of the bowl which displaces the less dense materials inwardly toward the rotational axis of the centrifuge. Contrastly, perforate basket centrifuges are typically used to separate materials based on differences in particle size rather than density; and, in such centrifuges, the centrifugal forces generated by the rotating centrifuge serve to throw all of the material outwardly against a perforate screen of a given mesh size. The particles smaller than the mesh size then pass outwardly through the screen while the particles larger than the mesh size collect on the screen for subsequent removal.
In the recycling of materials such as paper pulp, special problems are presented which can be uniquely solved using a combination of imperforate and perforate basket centrifuges. Specifically, all paper pulp recycling processes add water which necessarily requires that one or more of the steps in the process be a dewatering one. Such dewatering can be accomplished by using conventional drum filters, disc filters, or presses. However, due to the manner of their operation, these techniques tend to trap contaminating solids such as fillers and ink balls in the dewatered pulp. In contrast to these dewatering techniques, the use of a perforate basket centrifuge has been found to have several advantages in that it can not only remove more of the contaminating impurities but also can do so without using high contact pressures that can result in creating undesirable clumps in the pulp. A particularly successful perforate basket centrifuge for this purpose employs a perforate screen with a rotating screw conveyor positional interiorly of it. The conveyor flights of the screw rotate in the same direction as the perforate screen but at a different speed. In operation, the water and contaminants are flung outwardly through the perforate screen while the pulp fibers collect on it and are advanced toward the discharge end of the centrifuge by the rotating screw conveyor. In doing so, bundles of the fibers become trapped in the spaces between the conveyor flights and the perforate screen; and, as the conveyor rotates, the advancing fiber bundles become a wiping media which removes other fibers trapped across and within the holes of the perforate screen. The rotating conveyor also causes the fiber bundles to roll as they are advanced toward the discharge end of the centrifuge thereby enhancing the separation of the water and contaminants.
Although perforate basket centrifuges have been found to have excellent performance characteristics in the dewatering and contaminant separation of recycled materials such as paper pulp, they can be damaged by hard objects such as metal and glass. Consequently, it is highly desirable that such objects (which can include baling wire, metal strapping, paper clips, staples, glass, and sand) be removed from the pulp before it enters the perforate basket centrifuge otherwise damage can be done to the perforate screen. Techniques using pulper extractor plates, traps, magnets, and cyclone cleaners are only partially successful. In theory, a continuous feed, solid bowl centrifuge with an intermittent or batch discharge through its rim (other than nozzle or fixed orifice discharge means which are not capable of passing large objects such as baling wire, metal strapping and paper clips) is ideally suited to remove such objects. However, all presently known designs are both impractical and inefficient. For example, one known design mounts a rather heavy and complicated rim opening mechanism for rotation with the centrifuge. Understandably, this not only creates dynamic imbalance problems but also presents the inherent problem of transferring power to the rapidly rotating opening mechanism. Another problem with this design and all other known designs is that the rim is intermittently opened by moving portions of the solid bowl apart in a direction parallel to the rotational axis. Since these portions invariably have surfaces perpendicular or inclined to the rotational axis, the reclosing of the rim must then be done against the pressure of the centrifuged material which, in most cases, is quite substantial. Further, the angled surfaces create problems in keeping the bowl closed as the pressures generated within the centrifuge continuously tend to open the bowl. Consequently, the mechanism for closing the bowl and keeping it closed must likewise be quite substantial and capable of generating rather large forces. The end result in these known designs is an impractical and inefficient solid bowl centrifuge which cannot effectively be used with a downstream, perforate basket centrifuge.
It was with these problems in mind that the continuous feed, solid bowl centrifuge of the present invention was developed and particularly adapted for use alone and in combination with a continuous feed, perforate basket centrifuge. With the solid bowl centrifuge of the present invention and in its preferred environment, junk or tramp metal (e.g., baling wire, staples, paper clips) as well as glass and sand can be effectively and efficiently removed from recycled material such as paper pulp prior to its entry into a perforate basket centrifuge for further processing.